This invention concerns a passive surge control method for compression systems, and the relative device, able to reduce or eliminate the instabilities which are created in particular operating conditions.
The invention is applied particularly in the control of industrial turbo-compressors of an axial or centrifugal type.
In the field of compression systems, it is well-known that the operating field, in terms of pressure-flow of the operating fluid, is limited by phenomena of low frequency dynamic instability which occur under certain conditions.
It is well-known that for low values of fluid flow and high values of fluid pressure there is a zone of fluid dynamic instability, called surge zone, wherein the operating condition of the system becomes unacceptable, as it may entail considerable reductions in productivity and performance of the compressor, and also structural damage to the plant.
Working in the surge zone may also be extremely dangerous and unacceptable, for example in the case of gas turbines for use in aeronautics. In certain cases, the instability may also be associated with a flow-back of the fluid towards the compressor.
To overcome this problem, the most common practice has generally been to keep the operating point of the system sufficiently far from the limit defined as the so-called xe2x80x9csurge linexe2x80x9d.
Automatic systems have therefore been provided which activate a control member, for example a breather valve, when it is found that the system is approaching the surge line to an extent considered dangerous.
These systems, however, negatively affect the efficiency and performance of the system in that they do not allow it to function in zones which are near the surge line but outside the instability zone, where the maximum values of performance and pressure gap are found.
Therefore, more recently, control systems have been proposed which are able to modify the dynamic behaviour of the compression system in order to effectively amplify the stable operating field of the system and to allow, within certain limits, to reduce the amplitude of the surge zone wherein the system is prevented from working.
A first type consists of active instability control systems, which use a sensor suitable to monitor the value of a representative parameter of the state of the system (pressure, flow, or otherwise) and an actuator able to intervene in feedback in a closed loop pattern.
Although they are valid in most cases, such systems, of the type with a control logic outside the system, have the disadvantage that, in some situations, their flexibility is reduced and their capacity to adapt to all operating situations is limited.
As an alternative to active control systems, passive control systems have been proposed; these consist of a device of the mass-spring-damper type coupled in an aeroelastic manner with the compression plant and suitable to react in an intrinsic and adaptive fashion to the stresses which tend to take the system into conditions of instability.
With this approach, it is the structural feedback of the system which reacts to the causes of instability, rather than an outside controller which acts according to a pre-determined command logic.
However, since this system has to be able to absorb low frequency oscillations, the elasticity constant of the spring consequently has to be very low and therefore not compatible with conventional elastic elements of a mechanical type.
Therefore, in various documents it has been proposed to use a volume of gas as an elastic element. One such device, described in the article xe2x80x9cDynamic Control of Centrifugal Compressor Surge Using Tailored Structuresxe2x80x9d by D. L. Gysling et al., taken from Transactions of the ASME, October 1991, vol. 113, pages 710-722, provides to place an auxiliary chamber (auxiliary plenum) in contact with the main chamber (plenum) of the compression plant.
The two plenums are separated by a movable wall, which functions as the mass, constrained to an element which functions as a damper, and the movement of the movable wall is conditioned by a volume of gas in the auxiliary plenum which functions as a spring.
With this system, the low-frequency oscillatory disturbance which is generated in the main plenum and which leads to instability of the system is absorbed and dissipated thanks to the elastic movement of the movable wall and the damper connected thereto.
This system has been tried successfully on a small centrifugal compressor and has shown its efficiency and operating validity.
However, when this teaching is transferred to studies on applications of greater dimensions, more interesting and more common from an industrial point of view, it has been found that to ensure an efficacious control the volume of the auxiliary plenum has to assume excessive dimensions, absolutely incompatible with the constraints imposed by any industrial plant.
Therefore, even if the solution proposed appears valid and interesting from a theoretical point of view, in practical application it has numerous limits and is therefore inapplicable in most cases.
The present Applicant has tried and embodied this invention to solve the problems concerning passive control systems for compression plants, and to obtain further advantages as shown hereinafter.
The invention is set forth and characterized in the respective main claims, while the dependent claims describe other characteristics of the main embodiment.
The purpose of the invention is to achieve a method, and the relative devices, to carry out a passive control of the operating instabilities which occur in compression plants and systems, able to be applied substantially to every type of industrial plant with limited costs and with considerable operating efficiency and flexibility.
The method according to the invention provides to couple the compression system with an elastic system based on a column of liquid oscillating under the action of gravity and a suitable damper device.
To be more exact, the method according to the invention provides to use a liquid column whose surface (meniscus) is arranged in contact, either directly or indirectly, with the inner volume of the plenum of the compression system.
The liquid column communicates, through a suitable aperture or throttle, with a liquid volume whose meniscus is subject to a desired pressure, for example, atmospheric pressure.
With this embodiment, we obtain a self-adapting passive control system wherein the mass of liquid constitutes the inert element, the force of gravity determines the elastic return action and the connection aperture functions as a damper able to dissipate the oscillations which are generated in the plenum of the compression system under conditions of instability.
In fact, these oscillations determine a dynamic reply in the liquid column which is associated with the absorption of the non-stationary energy possessed by said oscillations and allows them to be dissipated through a suitable damper element. In this way, the instability can be eliminated.
When a condition of stability has been restored, the level of the two meniscuses of the two communicating liquid columns differs due to the head corresponding to the differential pressure.
The correct application of this device is conditioned by the suitable choices of the operating parameters, such as: mass of the liquid, areas of the meniscuses, height of the liquid column and entity of damping.
A device able to achieve the method according to the invention comprises at least a first container of a liquid whose meniscus is in contact with the inner volume of the plenum of the compression system.
In the preferential embodiment of the invention, the device and the plenum are connected by means of a tube connected at one end to the inner volume of the plenum and at the other end to the inner volume of said first container.
This embodiment allows to obtain a sufficiently ample surface at the meniscus, and thus able to efficiently perform the function of compensating the oscillations and instabilities of the plenum without requiring a bulky structure incompatible with the requirements of industrial plants.
Moreover, with this embodiment the volume of the plenum is not increased too much, and therefore no further encouragement is given for instabilities to occur.
The first container is immersed inside a second container, filled with liquid, with which it communicates by means of at least an aperture able to produce a loss of load.
In a possible embodiment, these connection apertures are adjustable in size so as to allow the control system to be set according to the specific requirements and the operating conditions of the compression system to which the control system is applied.
In one embodiment of the invention, the position of the first container inside the relative second container can be adjusted in order to define a specific level of immersion; when this is varied, the characteristics of the control device are varied.
According to another variant, in the case of high working pressures in the compression system, there is a system to reduce the pressure between the volume of the plenum and the meniscus of the liquid column.
This embodiment allows to obtain an efficient action to control the instability of the system without requiring the use of a column of water of an excessive height incompatible with practical applications.